1. Field of the Invention
This invention relates to a shift control system of a hydraulic, continuously variable transmission for an automotive vehicle.
2. Prior Art
In recent years, continuously variable transmissions (CVT""s), which control speed ratios steplessly, have attracted attention for their characteristics of enabling speed-change shocks to be avoided and for providing superior fuel economy CVT""s for use in automotive vehicles have been being developed vigorously.
Generally, the speed ratio of CVT""s is controlled by controlling the hydraulic pressure.
For example, in the case of a belt-type CVT, the power produced by an engine is transmitted via a belt from a primary pulley to a secondary pulley, the pulleys being rotating elements of the CVT. Normally, a gripping force is applied to the belt by a hydraulic pressure (hereinafter called xe2x80x9cthe line pressurexe2x80x9d) made to act on a hydraulic actuator for the secondary pulley, which is set in accordance with basic variables such as the torque being transmitted from the engine. The control of the speed ratio of the transmission is carried out through regulation of a hydraulic pressure (hereinafter called xe2x80x9cthe primary pressurexe2x80x9d) which acts on a hydraulic actuator for the primary pulley.
In the case of this kind of belt-type CVT, this speed ratio control (hereinafter called xe2x80x9cshift controlxe2x80x9d) is generally carried out by feedback-controlling the speed of rotation of the primary pulley. That is, shift control is carried out by setting a target speed of the primary pulley on the basis of the speed of the vehicle and a throttle angle or the like and controlling the primary pressure acting on the primary pulley so that the actual speed of the primary pulley approaches this target speed.
However, generally, when the rotational speed becomes low, detecting the rotational speed with a speed sensor becomes difficult. Consequently, when the vehicle is traveling extremely slowly or is stationary, because detection of the speed of the primary pulley is problematic, it is not possible to execute proper feedback control of the primary pulley""s rotational speed. Instead the speed ratio of the transmission is controlled to a speed ratio for minimal vehicle speeds (hereinafter called xe2x80x9cthe minimum speed ratioxe2x80x9d) by open loop control wherein a predetermined pressure is applied to the primary pulley.
However, with this open loop control, because its precision is poor, the following kinds of problem arise:
[1] When the primary pressure is too high, such as at times of travel in traffic jams, the transmission tends to gradually shift upwardly. Consequently, the response of the vehicle upon accelerating from a standstill deteriorates.
[2] When the primary pressure is too low, such as at times when accelerating immediately after braking suddenly, there is a risk of the speed ratio being at an intermediate level, it may not then be possible for the input torque to be transmitted, and slippage of the belt may occur. Also, the responsiveness of the transmission upshifting which accompanies an increase in the vehicle speed after the vehicle accelerates may deteriorate.
Another method of hydraulic pressure control of a hydraulic CVT, besides the speed feedback control and open loop control mentioned above, is the so-called xe2x80x9cpressure feedback control.xe2x80x9d In pressure feedback control, the actual value of the hydraulic pressure applied to the primary pulley (hereinafter xe2x80x9cthe actual primary pressurexe2x80x9d) is detected; a target value of the hydraulic pressure acting on the primary pulley (hereinafter xe2x80x9cthe target primary pressurexe2x80x9d) is set; and the hydraulic pressure control means of the primary pulley is feedback-controlled so that the actual primary pressure approaches the target primary pressure.
However, because, for example, immediately after engine start-up the line pressure is low, it may sometimes happen that the hydraulic cylinder of the primary pulley is not filled with operating fluid. In this case, when pressure feedback control is used for the hydraulic pressure control of the primary pulley, the problem arises that the primary pressure repeatedly overshoots and undershoots and shift control becomes unstable, and satisfactory control cannot be accomplished.
Also, when there is a failure of a hydraulic pressure sensor (xe2x80x9chydraulic pressure detecting meansxe2x80x9d) detecting the actual primary pressure, pressure feedback control itself becomes impossible, and problems arise such as the primary pressure being insufficient and the belt consequently slipping.
It is, therefore, an object of the invention to provide a shift control system of a hydraulic CVT for a vehicle with which, even when the vehicle is traveling extremely slowly or is stationary, it is possible to control the shift ratio certainly to a target value (for example, to a minimum shift ratio) while maintaining a required primary pressure.
fit is another object of the present invention to provide a shift control system of a hydraulic CVT for a vehicle with which, even when the vehicle is traveling extremely slowly or is stationary, it is possible to control a shift ratio using pressure feedback control while maintaining a required hydraulic pressure. It is also possible to control the shift ratio even when a hydraulic pressure applied to a rotating element of the CVT is insufficient or when there is a failure of hydraulic pressure detecting means.
To achieve these and other objects, in a shift control system of a hydraulic CVT for a vehicle, in a first aspect of the invention when the vehicle is not in a substantially stationary state (stationary or traveling extremely slowly), hydraulic pressure control means for controlling the behavior of a rotating element of the CVT with a hydraulic pressure is controlled by speed feedback control means so that the actual speed of the rotating element approaches a target speed set on the basis of the vehicle speed and the engine load. When the vehicle is in a substantially stationary state, the hydraulic pressure control means is controlled by pressure feedback control means so that the hydraulic pressure applied to the rotating element approaches a set target pressure.
As a result, even when the vehicle is traveling extremely slowly or is stationary, the shift ratio can be controlled certainly to a target value while maintaining the hydraulic pressure an optimal value.
Preferably, when the vehicle is substantially stationary and it is determined that the state of the transmission is such that pressure feedback control is not possible, control of the hydraulic pressure control means is switched from pressure feedback control executed by the pressure feedback control means to open loop control executed by open loop control means. Preferably, it can be determined that pressure feedback control is not possible when a failure of actual pressure detecting means for detecting the hydraulic pressure acting on the rotating element or when the actual pressure applied on the rotating element is below a reference pressure.
It can be inferred that the actual hydraulic pressure detecting means has failed when the detection value from the actual hydraulic pressure detecting means is outside a predetermined range.
In a shift control system of a hydraulic CVT for a vehicle, according to a second aspect of the invention, the actual value of the hydraulic pressure applied to a rotating element of the CVT is detected; a target value of the hydraulic pressure applied to the rotating element is set; and hydraulic pressure control means controlling the rotating element is controlled using pressure feedback control so that the actual value of the hydraulic pressure acting on the rotating element approaches the target value. Also, at this time it is determined whether the state of the transmission is such that this pressure feedback control is possible. When it is determined that pressure feedback control is not possible, control of the hydraulic pressure control means is switched from pressure feedback control to open loop control wherein a predetermined hydraulic pressure is made to be applied to the rotating element.
Preferably, it is determined that pressure feedback control is not possible when failure of actual hydraulic pressure detecting means detecting the hydraulic pressure being applied to the rotating element occurs or when the actual hydraulic pressure being applied to the rotating element is below a reference pressure. It can be inferred that the actual hydraulic pressure detecting means has failed when the detection value from the actual hydraulic pressure detecting means is outside a predetermined range.
More preferably, the vehicle speed is detected, and when the vehicle speed is above a predetermined value, control of the hydraulic pressure control means is switched from pressure feedback control or open loop control to speed feedback control wherein the hydraulic pressure control means is feedback-controlled so that the speed of the rotating element approaches a target speed set on the basis of the vehicle running state.
When the CVT is a belt-type CVT made up of a primary pulley, a secondary pulley and an endless belt passing around the two pulleys, the rotating element mentioned above can be taken to mean a primary pulley controlled by a supply of hydraulic pressure to a primary cylinder, and the hydraulic pressure detecting means mentioned above can be provided as a hydraulic pressure sensor detecting the hydraulic pressure supplied to this primary pulley.